Field of the Invention
The invention relates to microfluidic actuators for the pressurization, transport, mixing, and other processing of small volumes of liquid.
Description of the Related Art
Microfluidic actuators are small components—typically less than 1 cubic centimeter in displaced volume—that, while functionally similar to conventional hydraulic, electrohydraulic, and pneumatic actuators [1], typically employ design and operational principles specific to their comparatively small size. Microfluidic actuators may be categorized as mechanical or non-mechanical. Mechanical microfluidic actuators use moving diaphragms or other surfaces in a continuous or cyclical manner to pressurize a volume of fluid, which in turn can be used to do mechanical work. While the nominal throw of a conventional actuator is determined by the length of the cylinder, the throw of a microfluidic actuator is typically determined by the working fluid pressurization system. Long-throw microfluidic actuators with moving surfaces require valves. The valve seals are susceptible to obstruction and other failure modes, making this type of actuator not ideal for long-term use where reliability is important. Microfluidic actuators with active valves are expensive to produce, whereas microfluidic actuators with passive valves are limited in generating high pressure and high flow rate capacity.
Non-mechanical microfluidic actuators use electrical, magnetic, optical, chemical, or electrochemical means to pressurize a volume of working fluid, which in turn can be used to do mechanical work. Phase-change microfluidic actuators use heat or electrochemical effects to convert a liquid phase to a gas phase; the pressure associated with the phase change can be used do work. The maximum pressure generated through the phase change is typically small, limiting the applications of these actuators.
Electroosmotic (EO) microfluidic actuators are a type of non-mechanical microfluidic actuator. EO microfluidic actuators use body forces on mobile ions in the fluid phase of the electric double layer at a fluid-solid interface [2] to pressurize a fluid. The fluid is referred to as the EO working fluid. The pressurized EO working fluid can be used to do external mechanical work, i.e. moving an external mass over some distance. Pressurization of the EO working fluid is modulated through the controlled application of electrical fields within portions of the EO working fluid. Electrostatic body forces acting on mobile ions in the fluid phase of electric double layers at interfaces within stationary, fluid-contacting solid structures create pressure gradients within the EO working fluid.
EO devices incorporate electrodes for generating electrical fields which create body forces on mobile ions of the electric double layer. Some EO devices use aqueous solutions as working fluids. When the electrodes are polarized by a battery or other electrical potential source, continuous electrical current can flow through the electrolytic system formed by the electrodes, the aqueous, and the electrical potential source. Continuous current flow can be supported by oxidation-reduction reactions at the electrode-aqueous interface and ionic charge transport within the bulk aqueous.
Because EO microfluidic actuator operation is electric double layer-dependent, the shape and composition of the fluid-contacting solid structures are primary determinants of actuator performance parameters like maximum pressure, response time, and throw. Many previously designed EO devices have incorporated EO flow generating structures of porous polymer layers and silica beads packed between frits [3]. While these designs produce high maximum pressures, they can require high operating voltages and the tortuous path for fluids through the bead bed limits power transduction as a function of apparatus volume and results in characteristically low flow rates [4], [5].
Many previously designed EO devices incorporate one or more approximately rectangular cross-section channels with the two cross-sectional dimensions on the same order. These devices generally do not generate sufficient fluid power to be useful for doing mechanical work on an external mass, either because the volumetric flow rate is limited by the small total cross-sectional area or because the pressure generation is limited by the high ratio of the cross-sectional dimensions to the electric double layer characteristic thickness.
Some previously designed EO devices incorporate one or more approximately rectangular cross-section channels with one cross-sectional dimension between 3 and 10 microns and the other cross-sectional dimension much larger [6]. These devices based on slit-like channels can generate appreciable fluid power, but are difficult to fixture and load because of the large difference in the in-plane dimensions.
Other previously designed EO microfluidic actuators have used one-dimensional arrays of long, narrow, closely spaced interstices between a series of slat-like structural elements. Some configurations have a smaller cross-sectional dimension of the interstices between 3 and 10 microns and a large cross-sectional dimension between 50 and 250 microns [7]. This configuration has the high fluid power generation capability of EO devices with one or a small number of slit-like channels described above, but can be more readily integrated with other microfluidic components. For example, these devices can be built into plastic cartridges for analyzing blood to characterize genomic material contained therein [8]. The ratio of the large cross-sectional dimension to the small cross-sectional dimension is referred to as the interstice aspect ratio; the ratio of the small to the large cross-sectional dimensions of the EO flow area is referred to as the flow area aspect ratio. The reported devices have had interstice aspect ratios of approximately 20 or lower and flow area aspect ratios of more than 5.
Actuator throw, or the amount of liquid that can be moved through the apparatus, is an important determinant of the types of applications for which an actuator can be used. An EO pump has been reported incorporating a solid structure consisting of arrays of holes in silicon [9]. This design was limited in actuator throw as a function of apparatus volume and maximum pressure.
Accordingly, conventional microfluidic actuators, including conventional EO microfluidic actuators, are limited in throw, response time, maximum pressure, and suitability for integration with other microfluidic components. The present invention addresses these and other shortcomings of the prior art.